10

The shear in the beam may be obtained by assuming that floor loads act according to the 45° tributary areas of each respective beam. Additional shear from lateral loads and the direct loads on the beam should be added on. The shear design of the beam then follows the procedure presented in Section 7.12. Shear stresses in the floor slab are generally low, but they should be checked. The strip method, which approximates the slab shear by assuming a unit width of slab strip over the panel, may be used to estimate the shear force in the slab.

7.13.2 Flat Plates

Floor systems without beams are commonly referred to as flat plates, (see Figure 7.15). Flat plates are economical and functional because beams are eliminated and floor height clearances are reduced. Minimum thicknesses of flat plates are given in Table 7.8 and should not be less than 5 in. The structural design procedure is the same as for flat slab with beams, presented in the previous sections, except that for flat plates a1 = 0. Refer to Section 7.13.1.2 for the static moment calculation. For the exterior span the distribution of the static moment is given in Figure 7.14. Table 7.10 and Table 7.11 provide the application for moment assignments to column strips.

7.13.2.1 Transfer of Forces in slab-column connections

An important design requirement of the flat plate system is the transfer of forces between the slab and its supporting columns (see Figure 7.14 and Figure 7.16). This transfer mechanism is a complex one. The accepted design approach is to assume that a certain fraction of the unbalanced moment Munb in the slab connection is transferred by direct bending into the column support. This gf fraction is estimated to be